I've mostly got by alright with the smaller of the ventura bags, although a mate of mine has the larger one for his 'blade and he does then have the option to take more than two pairs of spare keks
Go for the biggie and you'll prob not regret it. Besides, its as safe as houses once on the rack and big or small, its all the same aesthetically.
No help on the GPS, Andy... Ive a very tatty map of France that's served me well though!!!!